Concrete reinforcement bars, known as rebars, are used as their name expresses, i.e., to reinforce concrete structures.
Concrete has poor tensile strength; accordingly, good engineering practice mandates the use of rebars in applications where the tensile strength of the concrete is inadequate and requires supplementation.
The rebars now in widespread use are made by a well-known hot rolling technique. They are round in transverse section and thus present a minimal surface area to the concrete which must adhere thereto. They have a tensile strength of about 40,000-60,000 p.s.i. Accordingly, conventional rebars are at least adequate to fulfil their intended unambitious purpose. Their relatively low tensile strength and their other uninspiring statistics are a result of the hot rolling process by which they are made.
A common problem with conventional rebars (other than their low tensile strength, high weight and minimal surface area) is their tendency to slide out of position after concrete has been poured around them, i.e., after they have been embedded in the concrete.
This longitudinal or axial slip may be countered by forming upwardly projecting ridge members on the outer surface of the rebar.
Concrete weights about one-tenth of a pound per cubic inch, whereas rebars, being made of steel, weigh about three-tenths of a pound per cubic inch. Thus, rebar-reinforced concrete is quite heavy. Perhaps more importantly, since steel and concrete expand and contract at different ratios in response to heating and cooling, respectively, rebar-reinforced concrete will crack during periods of rapid temperature changes more severely than non-reinforced concrete.
There is a need, therefore, for a machine that can make rebar that is higher in tensile strength yet lighter in weight than conventional rebar, that makes rebars having enhanced non-slip properties, and which makes rebars that, when embedded in concrete, will cause appreciably less cracking than rebars made with the machines of the prior art.
All of the earlier rebar-making machines known to the present inventor produce rebars by a hot-rolling process, as aforesaid. Hot rolling results in rebars of low tensile strength, as aforesaid, and wastes steel. Patents disclosing hot rolling machines and methods, when considered one at a time or as a whole, neither teach nor suggest to those skilled in the art of rebar-making machines how a machine could be built that would produce rebars not subject to the limitations of conventional hot-rolled rebars.
All of the prior patents, in the field of rebars, known to the present inventor, are as follows: U.S. Pat. Nos. 4,229,501 to Kern (1980); 4,119,764 to Mizuma et. al. (1978); 3,979,186 to Mizuma (1976); 3,561,185 to Finsterwalder et. al. (1971); 3,415,552 to Howlett (1968); 3,335,539 to Soretz (1967); 3,312,035 to VanKoot (1967).